1. Field of the Invention
The present invention relates to a method for producing a semiconductor optical device.
2. Description of the Related Art
Japanese Unexamined Patent Application No. 2001-320136 (Patent Literature 1) discloses a technique regarding a quantum cascade (QC) laser (hereinafter, also referred to as a “QC laser”) capable of generating pico-seconds pulses of mid-infrared light. In this laser, light emission due to inter-subband transitions of electrons in an active region included in a core region is used. The QC laser has a large nonlinear refractive index resulting from the inter-subband transitions of electrons. The Kerr-lens mode-locking of the QC laser is provided by using the index nonlinearity of the intersubband transition.
The intracavity aperture required to convert the QC self-focusing mechanism into a loss modulation is provided by a QC laser waveguide that is characterized by: (1) an optically highly lossy layer, separated from the semiconductor material by a relatively thin dielectric layer; and (2) a relatively long laser waveguide. In one embodiment of the Patent Literature 1, the highly lossy layer used for mode coupling is made of metal and is configured to also form one of the electrical contacts for the laser device. Furthermore, the QC laser includes a mesa structure in which the core region is included.